The team at Harvard tested 662 men who were enrolled at the Massachusetts General Hospital Fertility Center between 2000 and 2017. From the pool, they collected 1,143 semen samples.

The 365 men who reported having ever smoked pot had “significantly higher sperm concentration” — precisely 63 million sperm per milliliter of semen — compared to the 297 who have never taken a hit, which averaged 45 million/mL.

There were no significant differences in sperm counts between current and former marijuana smokers.

Researchers adjusted the findings for factors that could affect the sperm counts, like age, abstinence time, smoking, consumption of coffee, alcohol and cocaine.

The results were surprising — and show how much more there is to learn about the link between marijuana use and fertility, the scientists said.

“These unexpected findings from our study highlight that we know too little about the reproductive health effects of cannabis and, in fact, of the health effects in general, to make strong statements about the impact of cannabis on health, with the possible exception of mental health,” said lead researcher Jorge Chavarro, an associate professor of nutrition and epidemiology at Harvard, in a statement. “We know a lot less than we think we know.”

The study also found that among the men who had ever smoked pot, “those who smoked it more often had testosterone levels an average of eight nanograms per decilitre higher than those who used it less often.”

Dr. Feiby Nassan, one of the authors of the study, said men with higher testosterone levels are more likely to engage in risky behavior, like drug use, which could be behind the study’s finding of higher sperm counts.

“The relations we see between cannabis smoking, sperm counts and testosterone levels are because men with higher testosterone, within normal levels, have higher sperm counts and are more likely to smoke cannabis,” she said.

A normal sperm count is at least 15 million/mL, according to the World Health Organization.

Chavarro warned that the findings don’t necessarily mean that smoking pot increases the chances of making a baby.

“An equally important limitation is the fact that most of the data were collected while cannabis was illegal in Massachusetts, so it is difficult to know to what extent men may have under-reported use of cannabis because of social stigma or potential consequences related to insurance coverage for infertility services,” he said.